Climate variability, snow, and physiographic controls on storm hydrographs in small forested basins, western Cascades, Oregon

Abstract

Abstract:Large floods are often attributed to the melting of snow during a rain event. This study tested how climate variability, snowpack presence, and basin physiography were related to storm hydrograph shape in three small (<1 km 2 ) basins with old-growth forest in western Oregon. Relationships between hydrograph characteristics and precipitation were tested for approximately 800 storms over a nearly 30-year period. Analyses controlled for (1) snowpack presence/absence, (2) antecedent soil moisture, and (3) hillslope length and gradient. For small storms (<150 mm precipitation), controlling for precipitation, the presence of a snowpack on near-saturated soil increased the threshold of precipitation before hydrograph rise, extended the start lag, centroid lag, and duration of storm hydrographs, and increased the peak discharge. The presence of a snowpack on near-saturated soil sped up and steepened storm hydrographs in a basin with short steep slopes, but delayed storm hydrographs in basins with longer or more gentle slopes. Hydrographs of the largest events, which were extreme regional rain and rain-on-snow floods, were not sensitive to landform characteristics or snowpack presence/absence. Although the presence of a snowpack did not increase peak discharge in small, forested basins during large storms, it had contrasting effects on storm timing in small basins, potentially synchronizing small basin contributions to the larger basin hydrograph during large rain-on-snow events. By altering the relative timing of hydrographs, snowpack melting could produce extreme floods from precipitation events whose size is not extreme. Further work is needed to examine effects of canopy openings, snowpack, and climate warming on extreme rain-on-snow floods at the large basin scale.

Extreme flood sensitivity to snow and forest harvest, western Cascades, Oregon, United States

“…Precipitation, air temperature, snow water equivalent, and snowmelt lysimeter data were obtained from meteorological stations (http://andrewsforest.oregonstate.edu/data/abstractdetail.cfm?dbcode = MS001&topnav = 135) (Figures 2 and 3). Small basin peak discharge events were selected from continuous records using an automated selection procedure based on stage height changes, and the resulting peak discharges were matched by date and time with events at other basins following procedures in the study by Jones and Grant [1996], Jones [2000], and Perkins and Jones [2008]. Most peak discharges were matched within ±12 h, but some were up to ±36 h apart.…”

“…Most peak discharges were matched within ±12 h, but some were up to ±36 h apart. For each peak discharge, event precipitation was determined using an automated peak discharge identification procedure and the precipitation record from the site nearest the stream gage (Figure 2) following procedures in the study by Perkins and Jones [2008]. In automated procedures, points in the continuous streamflow record were identified as peak discharges based on criteria for changes in stage height; these were corroborated using the Get‐PQ routine that identifies peak discharges using stage height and precipitation timing [ Dripchak and Hawkins , 1992].…”

“…Precipitation type (rain, snow) varies with elevation and day of year. In the transient snow zone (400–800 m, primet, cs2met) snowpacks rarely last more than 1–2 weeks; in contrast, in the seasonal snow zone (>800 m), snowpacks persist from November to late April (cenmet) or June (vanmet, uplmet) (Figure 4b) [ Harr and McCorison , 1979; Perkins and Jones , 2008]. Rain shadow effects reduce winter (December–February) streamflow at the small high‐elevation basin (WS 8) relative to low‐elevation basins (WS 2, WS 9), but snowpack storage and melt augment spring (March–June) streamflow from basins draining the seasonal snow zone (WS 8, Lookout) relative to those in the transient snow zone (WS 9) (Figure 4c).…”

“…Climate warming is expected to affect transient (short‐duration, midelevation) snow more than seasonal snow [ Nolin and Daly , 2006; Cuo et al , 2009; Adam et al , 2009]. To account for climate effects on floods, studies increasingly separate rain‐on‐snow from rain peak discharge events [ Loukas et al , 2000; Merz and Blöschl , 2003; Perkins and Jones , 2008].…”

The impact of climatic conditions of spring flood formation on hydrograph shape of the Desna River

“…The objective method of diurnal hydrograph classification as an aid to exploring changes in the hydrological functioning of glaciered catchments over the ablation season was shown Hannah et al (1999). Perkins and Jones (2008) studied the influence of climatic factors on the formation of storm hydrographs in small forested basins. It should be noted that nowadays the issue of the influence of climatic factors on the spring flood hydrograph shape of lowland rivers is hardly studied.…”